[Haskell-beginners] applicative default structure
toad3k at gmail.com
Thu Dec 22 15:46:45 UTC 2016
I get a Num ambiguous constraint error as well, but it is merely not giving
you the rest of the errors.
Prelude> let a1 = pure (1::Int)
Prelude> let a2 = pure (1::Int)
Prelude> a1 == a2
• Ambiguous type variable ‘f0’ arising from a use of ‘a1’
prevents the constraint ‘(Applicative f0)’ from being solved.
Furthermore you can't compare an IO a with an IO a, so it will still cause
an error. But if you go:
Prelude>let a1 = 1
Prelude>let a2 = 1
Prelude>a1 == a2
Then it is totally fine with that.
On Thu, Dec 22, 2016 at 10:38 AM, Imants Cekusins <imantc at gmail.com> wrote:
> > what is the "default" structure if you don't specify any
> similar to:
> display::Show a => a -> String
> display = show
> fa::(Applicative f, Num a) => f a
> fa = pure 1
> f and a are *bounded* by Applicative and Num, so to say. No default. Or,
> it is typed however type is a bit broader than *
> > In this case it defaults f to IO and a to Int,
> does it though?
> Prelude> let a1 = pure 1
> Prelude> let a2 = pure 1
> Prelude> a1 == a2
> <interactive>:20:1: error:
> • Ambiguous type variable ‘a0’ arising from a use of ‘a1’
> prevents the constraint ‘(Num a0)’ from being solved.
> Beginners mailing list
> Beginners at haskell.org
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the Beginners